1. Field of the Invention
The present invention relates generally to stereo demodulators for FM stereo broadcasting, and more particularly, to a stereo demodulator which can always keep separation between right and left stereo signals the maximum.
2. Description of the Prior Art
In a matrix circuit in an FM stereo receiver in which right and left stereo signals are obtained by adding a stereo sum signal to a stereo difference signal, levels of the stereo sum signal and the stereo difference signal are adjusted so that good separation is obtained. The adjustment of the levels is made by, for example, the following method. A stereo composite signal including only a left stereo signal and not a right stereo signal is applied to a stereo demodulator, and the level of the stereo sum signal or the stereo difference signal to be applied to the matrix circuit is adjusted by a voltage divider circuit or the like such that an output level of an output terminal for the right stereo signal (right output terminal) of the matrix circuit in the stereo demodulator becomes zero. Thus, leakage from the left stereo signal to the right stereo signal can be made zero. In addition, leakage from the right stereo signal to the left stereo signal must be made zero. Therefore, a stereo composite signal including only the right stereo signal and not the left stereo signal is applied to the stereo demodulator, and the level of the stereo sum signal or the stereo difference signal is adjusted such that an output level of an output terminal for the left stereo signal (left output terminal) of the matrix circuit becomes zero. Thus, right and left stereo signals having good separation can be obtained.
FIG. 1 shows a conventional stereo demodulator in which separation is adjusted by adjusting levels of a stereo sum signal and a stereo difference signal applied to a matrix circuit. This stereo demodulator is described in, for example, Japanese Patent Publication No. 15580/1984.
In FIG. 1, a stereo composite signal as FM (frequency-modulation)-detected is applied to an input terminal 101. A stereo sum signal (L+R) in the stereo composite signal is applied to a matrix circuit 104 through a buffer amplifier 102 and an adjusting resistor 103. In addition, a stereo difference signal (L-R) in the stereo composite signal is demodulated by a difference signal demodulator 105, so that stereo difference signals (L-R) and -(L-R) whose phases are reverse to each other are applied to the matrix circuit 104. In the matrix circuit 104, the stereo sum signal L+R) and the stereo difference signal (L-R) are added so that a left stereo signal L is generated at a left output terminal 106, while the stereo sum signal (L+R) and the stereo difference signal -(L-R) are added so that a right stereo signal R is generated at a right output terminal 107. On the other hand, a series circuit of a variable resistor 108 and a resistor 109 is connected in parallel with the adjusting resistor 103. Separation can be adjusted by adjusting a value of the variable resistor 108.
However, in the above described method for adjusting separation, the resistance value of the variable resistor 108 must be manually adjusted while observing waveforms of output signals which appear at the left and right output terminals 106 and 107 of the matrix circuit 104, resulting in the inaccuracy of adjustment and the increase in the number of manufacturing processes. In addition, when the stereo demodulator is formed as an integrated circuit, the variable resistor 108 must be externally provided on an IC (integrated circuit), resulting in the increase in the number of parts externally provided and the increase in the number of pins externally provided, which is not preferable. Furthermore, when the initialization value of the variable resistor 108 deviates, separation is decreased. Additionally, the resistance value of the variable resistor 108 deviates due to the change in environment such as the change in temperature, the change with time and the change with age, so that the separation is deteriorated again.